1. Field of Invention
The present invention relates to a method of forming polymer-wood composite structures and additive systems for use therein.
2. Description of Related Art
For many years, thermoplastic polymers have been melt-mixed with cellulosic filler materials such as saw dust and extrusion molded to form composite “plastic wood” or “synthetic lumber” products (hereinafter generally referred to as “polymer-wood composites”). Structures (e.g., deck boards) formed of polymer-wood composites tend to be lighter in weight and significantly more moisture resistant than similarly sized structures formed solely of natural wood. In addition, polymer-wood composite structures can be formed from recycle streams of thermoplastic polymers and cellulosic fillers, which helps reduce the demand for natural wood and virgin polymer and thus aids in resource conservation.
The output rate determinative step in the production of polymer-wood composite structures is the rate at which such material can be extruded. If the extrusion rate is too high, the surface appearance of the resultant structure tends to be commercially unacceptable. In order to be commercially acceptable, the surface of a polymer-wood composite structure must be smooth, so as to approximate the surface of natural wood.
A variety of internal and external lubricants and/or release agents are used in production of polymer-wood composite structures in an effort to increase output rate. The most commonly used lubricant package in polymer-wood composites is a combination of a metal stearate, typically zinc stearate, and a synthetic wax, typically ethylene-bis-stearamide (hereinafter “EBS”) wax. This conventional lubricant package allows for an acceptable output rate and a commercially acceptable surface appearance.
While the use of a zinc stearate/EBS wax lubricant package does facilitate an increase in extrusion molding output rate, it also presents certain disadvantages. For example, there is a significant amount of scrap material generated during the production of polymer-wood composite structures. Ideally, this material would simply be reprocessed. However, scrap material containing zinc stearate/EBS wax cannot be reprocessed without creating an unacceptable surface appearance in the resulting polymer-wood composite structure. Moreover, the output rate provided by zinc stearate/EBS wax lubricant package is not optimal. Thus, there remains substantial room for improvement in the art.